1. Field of the Invention
The present invention relates generally to methods and systems for preventing corrosion of metal structures.
2. Discussion of the Background Art
In the construction of large metal structures, steel remains the economic choice of materials. Unfortunately, steel has a tendency to corrode over time.
A variety of methods for controlling corrosion have evolved over the past several centuries, with particular emphasis on methods to extend the life of metallic structures in corrosive environments. These methods typically include protective coatings which are used principally to upgrade the corrosion resistance of ferrous metals, such as steel, and some nonferrous metals, such as aluminum, and to avoid the necessity for using more costly alloys. Thus, they both improve performance and reduce costs. However, such protective coatings typically have several pitfalls.
Protective coatings fall into two main categories. The largest of these categories is the topical coating such as a paint, that acts as a physical barrier against the environment. The second category consists of sacrificial coatings, such as zinc or cadmium, that are designed to preferentially corrode in order to save the base metal from attack.
Cathodic protection and coatings are both engineering disciplines with a primary purpose of mitigating and preventing corrosion. Each process is different: cathodic protection prevents corrosion by introducing an electrical current from external sources to counteract the normal electrical chemical corrosion reactions whereas coatings form a barrier to prevent the flow of corrosion current or electrons between the naturally occurring anodes and cathodes or within galvanic couples. Each of these processes provided limited success. Coatings by far represent the most widespread method of general corrosion prevention (see Leon et al U.S. Pat. No. 3,562,124 and Hayashi et al U.S. Pat. No. 4,219,358). Cathodic protection, however, has been used to protect hundreds of thousands of miles of pipe and acres of steel surfaces subject to buried or immersion conditions.
The technique of cathodic protection is used to reduce the corrosion of the metal surface by providing it with enough cathodic current to makes its anodic disillusion rate become negligible (for examples, see Pryor, U.S. Pat. No. 3,574,801; Wasson U.S. Pat. No. 3,864,234; Maes U.S. Pat. No. 4,381,981; Wilson et al U.S. Pat. No. 4,836,768; Webster U.S. Pat. No. 4,863,578; and Stewart et al U.S. Pat. No. 4,957,612). The cathodic protection concept operates by extinguishing the potential difference between the local anodic and cathodic surfaces through the application of sufficient current to polarize the cathodes to the potential of the anodes. In other words, the effect of applying cathodic currents is to reduce the area that continues to act as an anode, rather than reduce the rate of corrosion of such remaining anodes. Complete protection is achieved when all of the anodes have been extinguished. From an electrochemical standpoint, this indicates that sufficient electrons have been supplied to the metal to be protected, so that any tendency for the metal to ionize or go into solution has been neutralized.
However, there is a strong divergence of opinion between the proponents of paint coatings and the proponents of cathodic protection. Proponents of "coatings only" are often on one side discounting the advantages of cathodic protection, claiming that a good, well applied coating is the only necessary protection for steel. On the other side, the proponents of cathodic protection often claim that any immersed or buried metal structure can best be protected by the installation of a well engineered cathodic protection system. There are many conditions under which one type of protection may be superior to the other. However, under most of the more commonly occurring conditions, the best conventional corrosion protection is actually a combination of both concepts. But even when the two concepts are combined, problems still occur.
Inorganic zinc coatings have functioned previously by allowing a limited sacrificial corrosion of the incorporated zinc to provide sufficient free electrons to preclude the removal of electrons from the underlying steel during the corrosion process. Under normal conditions of exposure in an industrial atmosphere, in the United States, a two mil coating could be expected to protect steel from corrosion for from four to six years depending upon the weather. Submerged in a salt water environment, the same coating would provide from one to two years of corrosion prevention to the underlying steel. When used to protect girder type highway bridges or automobile underbodies, inorganic zinc has proven less successful because the continuous contact with chloride ions and moisture accelerates the sacrifice of the metallic zinc in the coating and blisters off the various organic top coats.
The destruction of organic top coats over the inorganic zinc coatings has been particularly severe in those cases where impressed cathodic protection was attempted simultaneously. In general, the problem of top coating with organic top coats over inorganic zinc coatings has been the eventual intrusion of water through the organic coating that contacted the zinc and released sufficient hydrogen from the corrosion process to blister off the organic top coat. The destruction of organic top coats over inorganic zinc coatings has been particularly severe in those cases where impressed cathodic protection was attempted simultaneously. When impressed cathodic protection was applied to the system, the electric potential caused electroendomesis and blistered off the top coat even more quickly than when no current was applied.
In galvanic corrosion, those metals that have conducting or n-type semiconducting products (passive films, scales, and so forth) are at risk from the standpoint of localized attack caused by the ability of the surface films to support cathodic reactions and hence to provide a galvanic influence to the corrosion process. That is not to say that materials with nonconductant or p-type semiconducting films are not at risk. Aluminum is an obvious exception, as are results with very thin films (nickel and copper) that support electron transfer by tunnelling or surface states. It can be said, however, that the galvanic influence to localized corrosion, when it occurs in aqueous systems, requires a cathode material capable of supporting reduction of H.sup.+. This is most likely to be the case for n-type semiconductors, intrinsic or degenerate conductors or for very thin films.
The products of corrosion, especially with solids, fall under three different categories, based on their ability to serve as electrodes, these three categories being insulators, semiconductors, and conductors. The dividing line between categories is quite hazy and a particular oxide or sulfide may exhibit a range of conductivity depending on its degree of stoichiometry.
It has been previously shown that corrosion is generally the development of a galvanic couple between anodic (oxidizing) sites and cathodic (reducing) sites upon a metallic surface immersed in a conductive solution of ionizable compounds, such as seawater. This galvanic couple allows the transfer of electrons through the corroding metal from the ions formed by oxidation at the anodic sites to reducible ions at the cathodic sites. The overall result is that metal is converted to its various compounds at the anodes and reduction of various ions takes place at the cathodes, until all of the original metal is converted to a lower chemical energy state.